1. Field of the Invention
The present invention relates to an over-voltage protection circuit that protects a circuit from an over-voltage.
2. Description of the Related Art
Circuit elements used for a semiconductor integrated circuit (“IC”) may lose their normal functions when being applied with a voltage exceeding a breakdown voltage. Electronic devices operated by using external power supply, especially devices likely to be operated using power supply for emergency using batteries or low quality USB (Universal Serial Bus) power supply, have possibility to be applied with a high voltage deviating from the rated voltage. Therefore, there is a need of an over-voltage protection circuit for protecting a circuit element from an over voltage.
[Patent Document 1] Japanese Patent Application (Laid Open) No. H9-219935
The applicant has recognized the below problems from a review of an over-voltage protection circuit that has a voltage protection function by using a zener voltage at its input stage. In the over-voltage protection circuit, where a power supply voltage is externally applied across an input terminal and the reference voltage terminal is grounded, an input diode is configured so that the cathode becomes the input terminal side and the anode becomes the reference voltage terminal side. When an over-voltage is applied to the input terminal, the power supply voltage is clamped to the zener voltage, thus protecting the circuit elements.
However, a large voltage is applied across the input diode in the forward direction when the external power supply is connected to the protection circuit to have the opposite polarity, and this may damage the reliability of the input diode. For example, a large voltage may be applied across the input diode in the forward direction if a socket is connected in wrong direction when using a USB power supply from self-made personal computer, or if a polarity of battery is reversed when using an emergency power supply using the battery. Accordingly, there is a need to take this case into consideration with respect to circuit design.